Many devices have been developed in an attempt to successfully monitor and detect the passage of persons from an unconfined area of predetermined size. For example, U.S. Pat. No. 3,478,344 to Schwitzgebel et al. describes a behavior supervision system including four spaced base transceivers communicating with mobile transceivers mounted to the belt of each of a plurality of persons to be supervised so that the four base units can pinpoint the location of each mobile unit and sound an alarm if one leaves a predetermined area. Another alarm system is disclosed in U.S. Pat. No. 3,947,832 which describes a miner's helmet containing a radio transmitter that will alarm a base unit whenever the miner enters a danger zone.
An object of the invention, therefore, is to provide a means for supervisory personnel at a base unit to become alerted when supervised personnel carrying a mobile unit leave a prescribed area.
Another object of the invention is to alert supervised persons carrying a mobile unit responsive to a base unit transmitter that they are straying from a predetermined prescribed area.